You have been captured by a mad king, he has placed you in a large field, he says you are free to go once you have calculated the area of the field.

He has left you with 1 pencil, 1 piece of paper, and 1 30cm ruler.

What is the quickest way to work out the area of the field, so you can go home to your family?

*NOTE, from any position inside the field you can see the entire perimeter, assume that the field has four straight sides, though the angles may all be different.*

*Also, the area of the field is large enough to make it impractical to measure with the ruler*